Implemented Choice.getError.
Removed test stubs for the ExtCore.Control.Choice module, because it no longer exists.
Renamed Choice.getChoice to Choice.getError.
Modified some tests to use Collection.assertEqual instead of assertEqual, so we get better error messages if the test fails.
Removed some optimizations from IntSet/IntMap/LongSet/LongMap; it's not clear that they're improving performance dramatically, and the code is more difficult to maintain.
Use 'invalidArg' instead of 'failwith'.
Implemented List.Cont.fold.